The present invention relates to a method of driving a plasma display panel. More particularly, the present invention relates to an art to improve the light emission efficiency of a plasma display panel.
A plasma display panel is a device in which mixed gases of such as Ne and Xe for discharge are filled in a space of about 100 xcexcm width between two glass substrates on which electrodes are formed, a voltage greater than the discharge start voltage is applied between the electrodes to cause a discharge to occur, fluorescent materials formed on the substrates are excited to emit light by the ultraviolet rays generated by the discharge, and it is expected to be a display device that has the possibility of realizing a large-sized full-color display apparatus because of its advantages in display area, display capacity, responsiveness, and so on. Moreover, a direct view type plasma display panel of 40 to 60 inches has been realized . This size has not been realized in other display devices currently. Since the plasma display panel has been disclosed, such as in EP 0762373 A2, and is widely known, a description is omitted here.
As mentioned above, a plasma display panel has many advantages but, concerning power consumption, it is inferior to a CRT, and further improvement is demanded although a practical level has been attained in brightness. In other words, the greatest obstacle of the plasma display panel lies in improvement in the light emission efficiency, and many proposals concerning this problem have been presented. There are many methods of improvement, such as in the materials or the manufacturing process of the panel, in the driving method, and so on. Among those methods of improvement in the driving method, there are some methods of improvement in which the sustain discharge is improved.
In Japanese Unexamined Patent Publication (Kokai) No. 58-21293, the art to improve the light emission efficiency, in which the Townsend discharge is caused to occur by applying a very narrow pulse of 1 xcexcs or less, particularly a pulse of a high voltage between sustain discharge electrodes in a plasma display of DC type in which electrodes are exposed into the discharge space, has been disclosed. Moreover, in Japanese Unexamined Patent Publication (Kokai) No. 7-134565, the art to improve the light emission efficiency of a plasma display of AC type utilizing the principle of the Townsend discharge, in which discharge electrodes are covered by dielectric materials, has been disclosed.
Furthermore, in EID98-101 (pp. 125-129) published by the Electronic Information Communication Conference, the art in which a narrow pulse of 1 xcexcs or less and about 180 V is applied to one of discharge electrodes and, to the other electrode, a long pulse of a low voltage is applied, has been disclosed.
Still furthermore, in Japanese Unexamined Patent Publication (Kokai) No. 11-65514 and Japanese Unexamined Patent Publication (Kokai) No. 10-333635, the art in which a mixed pulse of a narrow pulse of a high voltage and a wide pulse of a low voltage are applied to sustain electrodes has been disclosed.
It is generally known that the narrower the sustain discharge pulse to be applied between sustain electrodes is, the better the light emission efficiency is, and the lower the voltage of the sustain discharge pulse is, the better the light emission efficiency is in the range where the sustain discharge is caused to occur. The above-mentioned conventional art also utilizes these characteristics, but a problem is caused when the disclosed driving method is employed. It is necessary, for example, to increase the absolute voltage (simply referred to as voltage in some cases hereinafter) of the pulse in order to generate and maintain the sustain discharge by applying narrow pulses. If, however, sustain discharge pulses of a high voltage are applied, the voltage becomes almost as high as the discharge start voltage, resulting in reduction of the operating voltage margin and an occurrence of erroneous display.
In a more concrete example, the discharge start voltage of the AC type plasma display panel currently put to practical use is about 200 V to 230 V. In the plasma display panel, the voltage of the sustain discharge pulse and wall charges are controlled so that wall charges are formed on the electrodes of a lit cell and not formed on those of an unlit cell, a sustain discharge is caused to occur in the lit cell because the voltage of the wall charged are overlapped on the sustain discharge pulse and the discharge start, voltage is exceeded, and no sustain discharge is caused to occur in the unlit cell because no voltage of the wall charges is overlapped, when the address action is completed. If the voltage of a pulse is made to be 200 V in order to generate and maintain the sustain discharge by applying the narrow pulse, there exist some unlit cells in which a discharge is caused to occur without wall charges. If a discharge is not caused to occur when sustain discharge pulses are applied several times in the beginning of the sustain discharge period, some unlit cells that are contiguous to lit cells may be lit after repeated sustain discharges because the discharge start voltage in the unlit cell is lowered by the priming effect which is caused by such as the charged particles that fly from the contiguous lit cells, resulting in erroneous display.
Moreover, if the voltage of the sustain discharge pulses is lowered, a problem is caused in that the discharge is terminated halfway because the amount of charge that move between electrodes in the sustain discharge is small and the sustain discharge cannot be continued.
For the reason""s described above, it has been difficult to reduce the width of the sustain discharge pulse sufficiently and lower the voltage of the sustain discharge pulse, therefore, the improvement of the light emission efficiency has not been sufficient.
The object of the present invention is to realize a new method of driving a plasma display panel with high brightness and low power consumption by reducing the width of the sustain discharge pulse and by lowering the voltage of the sustain discharge pulse to further improve the light emission efficiency.
To realize the above-mentioned object, according to the present invention, wall charges different from those in the lit cell are left on the electrodes in the unlit cell after the reset period and the address period and before the sustain discharge period, and the sustain discharge period pulses are set asymmetrically, with these wall charges being taken into account. When the sustain discharge period pulse with a higher absolute voltage is applied, the wall charges in the unlit cell are made to act to decrease the absolute voltage so that the unlit cell is prevented from being lit. By this, in the cell (unlit cell) in which sustain discharges are not caused to occur, the absolute voltage of the sustain discharge pulse is cancelled even if it is high, therefore, the operating voltage margin can be kept wide and the range of the voltage application conditions to improve the light emission efficiency can also be widened.
For example, if the width of the sustain discharge period pulse is narrowed in the lit cell, the voltage that causes the sustain discharge to occur without fail can be applied because the absolute voltage of the sustain discharge pulse is high, and the improved effect of the light emission efficiency by narrowing the pulse width can be obtained. On the other hand, when the sustain discharge period pulse with a lower absolute voltage is applied, because the wall charges in the unlit cell serves to increase the absolute voltage, it is necessary to lower the voltage of the sustain discharge period pulse so that no discharge is caused to occur even if the voltage of the wall charges in the unlit cell is overlapped. Simultaneously, the pulse width is made longer because it is necessary to move sufficient amount of the wall charge to maintain the discharge.
Concerning the shape of the sustain discharge pulse, various modifications are available. Moreover, although the sustain discharges pulse is realized by the signals applied between the two electrodes, respectively, it is possible to modify the shape of the signals to be applied to each electrode.
There are also various methods to form different wall charges in the unlit cells. In one method, for example, wall charges of opposite polarity are left in the first and second electrodes in the reset period, and the wall charges in the unlit cell are maintained and wall charges of opposite polarity are formed in the lit cell in the address period.
In another method, the residual wall charges in the reset period are maintained in the lit cell in the address period, and wall charges of the polarity opposite to that of the residual wall charges in the reset period are formed in the unlit cell.